This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-69207 filed on Mar. 12, 2001.
The present invention relates to raindrop detection apparatus and method providing reliable raindrop detection regardless of conditions in which the raindrop detection apparatus is installed.
One application of a raindrop detection apparatus is for an automatic windshield wiper control device for a vehicle. This type of apparatus is generally installed on the interior surface of a front windshield of a vehicle. The apparatus includes a light emitting device and an optical raindrop detection device. Furthermore, the apparatus has an amplifying section, a light emitting device driving section, and a raindrop detection section.
The light emitting device emits light toward the front windshield. The optical raindrop detection device includes a photoreceptor which receives reflected light from the front windshield. An infinitesimal signal from the photoreceptor is amplified in the amplifying section. Under the condition in which no raindrop is present on the front windshield, an output from the amplifying section is fine adjusted to a predetermined target value (a reference value). The fine adjustment is made by changing a level of the signal from the photoreceptor. To change the level of the signal from the photoreceptor, light emission of the light emitting device is controlled by adjusting a power supplied to the light emitting device.
The fine adjustment to the output is made in a light emitting device driving section when the raindrop detection apparatus installed in a vehicle is turned on. Changes in the output from the amplifying section due to variation of components are accommodated so that the output is closely adjusted to the predetermined target value. The variation results from the light intensity characteristics of a photoreceptor and a light emitting device, the electrical characteristics of electrical circuits, the temperature characteristics of each component, and the installation instability of optics components.
The raindrop detection section determines the amount changed in the output from the reference value as a result of a raindrop on the front windshield.
As the raindrop detection apparatus is installed in various models of vehicles, it is required to be usable on various windshields having different thickness and curvature. Thickness and curvature, in other words, a refractive index and light transmittance of the windshields vary with models of vehicles. These variations make significant differences in the output because the reflected light from the windshield varies with models. Therefore, a large range adjustment capability is required for this type of raindrop detection apparatus in order to adjust the output to the predetermined target value.
If the apparatus has the large range adjustment capability, the output can be adjusted relatively close to the target value in the different conditions. However, configuration of the apparatus becomes complex, and the adjustment range is limited since it is defined by a light emitting capacity of the light emitting device.
Moreover, when improving a conversion accuracy of a digital-to-analog (D/A) converter, a signal-to-noise (S/N) ratio is traded off. To maintain reasonable S/N ratio, the conversion accuracy of the D/A converter cannot be improved as desired. As a result, the output cannot be adjusted within a desirable range.
The present invention therefore has an objective to provide a raindrop detection apparatus enabling wide range adjustment to an output from an amplifier section with relatively simple configurations.
The present invention has another objective to provide a method for a reliable raindrop detection regardless of conditions of the raindrop detection.
A raindrop detection apparatus of the present invention includes a light emitting device, and a photoreceptor. Furthermore, the apparatus has a light emitting device driving section, an amplifying section, and a control section. The light emitting device emits light toward an object on which a raindrop falls. The photoreceptor receives reflected light from the object. The driving section controls a power supply to a light emitting device. The amplifying section amplifies a signal from a photoreceptor. The control section controls operations of the driving section and the amplifying section, and detects a raindrop on the object.
The control section also has a first and a second adjustment members. The first adjustment member adjusts an output from the amplifying section to a predetermined target value by varying amplifying gains. The second adjustment member adjusts the output to the target value by varying a power supply to the light emitting device after the first adjustment is completed. The first adjustment is for roughly adjusting the output to the target value in steps, and the second adjustment is for adjusting the roughly adjusted output more closely to the target value.
As those members utilize different components and take care of different adjustment ranges, capability and adjustment range requirements for each adjustment are reduced. This permits the raindrop detection apparatus to have a relatively simple configuration while providing a wide range adjustment for the output from the amplifying section.